Principles of Operating Systems - I/O Structures and Storage 1

ICS 143 - Principles of Operating Systems

Operating Systems - ReviewProf. Nalini Venkatasubramaniannalini@ics.uci.edu

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What is an Operating System?

An OS is a program that acts an intermediary between the user of a computer and computer hardware.

Major cost of general purpose computing is software. OS simplifies and manages the complexity of

running application programs efficiently.

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Operating System Views

Resource allocatorto allocate resources (software and hardware) of the

computer system and manage them efficiently.

Control programControls execution of user programs and operation of

I/O devices.

Kernel The program that executes forever (everything else

is an application with respect to the kernel).

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Operating System Spectrum

Monitors and Small KernelsBatch Systems

• Polling vs. interrupt

MultiprogrammingTimesharing Systems

• concept of timeslice

Parallel and Distributed Systems• symmetric vs. asymmetric multiprocessing

Real-time systems• Hard vs. soft realtime

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Computer System Structures

Computer System OperationI/O StructureStorage Structure

Storage Hierarchy

Hardware ProtectionGeneral System ArchitectureSystem Calls and System ProgramsCommand Interpreter

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Operating System Services

Services that provide user-interfaces to OS Program execution - load program into memory and run it I/O Operations - since users cannot execute I/O operations

directly File System Manipulation - read, write, create, delete files Communications - interprocess and intersystem Error Detection - in hardware, I/O devices, user programs

Services for providing efficient system operation

Resource Allocation - for simultaneously executing jobs Accounting - for account billing and usage statistics Protection - ensure access to system resources is controlled

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Process Management

Process - fundamental concept in OSProcess is a program in execution.Process needs resources - CPU time, memory, files/data

and I/O devices.

OS is responsible for the following process management activities.

Process creation and deletionProcess suspension and resumptionProcess synchronization and interprocess communicationProcess interactions - deadlock detection, avoidance and

correction

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Process Concept

An operating system executes a variety of programs

batch systems - jobstime-shared systems - user programs or tasksjob and program used interchangeably

Process - a program in executionprocess execution proceeds in a sequential fashion

A process containsprogram counter, stack and data section

Process Statese.g. new, running, ready, waiting, terminated.

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Process Control Block

Contains information associated with each process

• Process State - e.g. new, ready, running etc.• Program Counter - address of next instruction to be

executed• CPU registers - general purpose registers, stack pointer

etc. • CPU scheduling information - process priority, pointer• Memory Management information - base/limit

information• Accounting information - time limits, process number• I/O Status information - list of I/O devices allocated

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Schedulers

Long-term scheduler (or job scheduler) - • selects which processes should be brought into the ready

queue. • invoked very infrequently (seconds, minutes); may be slow.• controls the degree of multiprogramming

Short term scheduler (or CPU scheduler) -• selects which process should execute next and allocates

CPU.• invoked very frequently (milliseconds) - must be very fast

Medium Term Scheduler• swaps out process temporarily• balances load for better throughput

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Process Creation

Processes are created and deleted dynamically

Process which creates another process is called a parent process; the created process is called a child process.

Result is a tree of processes e.g. UNIX - processes have dependencies and form a

hierarchy.

Resources required when creating processCPU time, files, memory, I/O devices etc.

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Process Termination

Process executes last statement and asks the operating system to delete it (exit).

• Output data from child to parent (via wait).• Process’ resources are deallocated by operating system.

Parent may terminate execution of child processes.

• Child has exceeded allocated resources.• Task assigned to child is no longer required.• Parent is exiting

– OS does not allow child to continue if parent terminates– Cascading termination

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Threads

Processes do not share resources well • high context switching overhead

A thread (or lightweight process) • basic unit of CPU utilization; it consists of:

– program counter, register set and stack space

A thread shares the following with peer threads:– code section, data section and OS resources (open files,

signals)

Collectively called a task.Heavyweight process is a task with one thread.

Thread support in modern systems User threads vs. kernel threads, lightweight processes1-1, many-1 and many-many mapping

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Producer-Consumer Problem

Paradigm for cooperating processes; producer process produces information that is

consumed by a consumer process.

We need buffer of items that can be filled by producer and emptied by consumer.

• Unbounded-buffer places no practical limit on the size of the buffer. Consumer may wait, producer never waits.

• Bounded-buffer assumes that there is a fixed buffer size. Consumer waits for new item, producer waits if buffer is full.

Producer and Consumer must synchronize.

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Interprocess Communication (IPC)

Mechanism for processes to communicate and synchronize their actions.

• Via shared memory• Via Messaging system - processes communicate without

resorting to shared variables.

Messaging system and shared memory not mutually exclusive -

– can be used simultaneously within a single OS or a single process.

IPC facility provides two operations.– send(message) - message size can be fixed or variable– receive(message)

Direct vs. Indirect communication.

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CPU Scheduling

Scheduling ObjectivesLevels of SchedulingScheduling CriteriaScheduling AlgorithmsMultiple Processor SchedulingReal-time Scheduling

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Scheduling Policies

FCFS (First Come First Serve)• Process that requests the CPU FIRST is allocated the CPU

FIRST.SJF (Shortest Job First)

• Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time.

Priority • A priority value (integer) is associated with each process.

CPU allocated to process with highest priority.Round Robin

• Each process gets a small unit of CPU timeMultiLevel

• ready queue partitioned into separate queues• Variation: Multilevel Feedback queues.

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Process Synchronization

The Critical Section ProblemSynchronization HardwareSemaphoresClassical Problems of SynchronizationCritical RegionsMonitors

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The Critical Section Problem

Requirements• Mutual Exclusion• Progress• Bounded Waiting

Solution to the 2 process critical section problem

Bakery AlgorithmSolution to the n process critical section problemBefore entering its critical section, process receives a

number. Holder of the smallest number enters critical section.

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Synchronization Hardware

Test and modify the content of a word atomically - Test-and-set instruction

function Test-and-Set (var target: boolean): boolean; begin Test-and-Set := target; target := true; end;

Mutual exclusion using test and set. Bounded waiting mutual exclusion using test and

set.

“SWAP” instruction

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Mutual Exclusion with Test-and-Set

Shared data: var lock: boolean (initially false)

Process Pirepeat

while Test-and-Set (lock) do no-op;critical section

lock := false; remainder section

until false;

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Bounded Waiting Mutual Exclusion with Test-and-Set

var j : 0..n-1; key : boolean;repeat waiting [i] := true; key := true;

while waiting[i] and key do key := Test-and-Set(lock);

waiting [i ] := false; critical section j := i+1 mod n; while (j <> i ) and (not waiting[j]) do j := j + 1 mod n; if j = i then lock := false; else waiting[j] := false; remainder section

until false;

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Semaphore

Semaphore S - integer variable• used to represent number of abstract resources.• Binary vs. counting semaphores.

Can only be accessed via two indivisible (atomic) operations

wait (S): while S <= 0 do no-op S := S-1;signal (S): S := S+1;

• P or wait used to acquire a resource, decrements count • V or signal releases a resource and increments count• If P is performed on a count <=0, process must wait

for V or the release of a resource.

Block/resume implementation of semaphores

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Classical Problems of Synchronization

Bounded Buffer ProblemReaders and Writers ProblemDining-Philosophers Problem

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Readers-Writers Problem

Shared Datavar mutex, wrt: semaphore (=1); readcount: integer (= 0);

Reader processwait(mutex); readcount := readcount +1; if readcount = 1 then wait(wrt); signal(mutex); ... reading is performed ... wait(mutex); readcount := readcount - 1; if readcount = 0 then signal(wrt); signal(mutex);

Writer Processwait(wrt); … writing is performed ... signal(wrt);

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Critical Regions

High-level synchronization constructA shared variable v of type T is declared as:

var v: shared T

Variable v is accessed only inside statement

region v when B do S

where B is a boolean expression.While statement S is being executed, no other

process can access variable v.

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Monitors

High-level synchronization construct that allows the safe sharing of an abstract data type among concurrent processes.

type monitor-name = monitor variable declarations procedure entry P1 (…); begin … end; procedure entry P2 (…); begin … end; . . . procedure entry Pn(…); begin … end; begin initialization code end.

Hoare vs. Mesa Monitors

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Deadlocks

System Model Resource allocation graph, claim graph (for avoidance)

Deadlock Characterization Conditions for deadlock - mutual exclusion, hold

and wait, no preemption, circular wait.

Methods for handling deadlocksDeadlock PreventionDeadlock AvoidanceDeadlock DetectionRecovery from Deadlock

Combined Approach to Deadlock Handling

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Deadlock Prevention

If any one of the conditions for deadlock (with reusable resources) is denied, deadlock is impossible.

Restrain ways in which requests can be madeMutual Exclusion - cannot deny (important)Hold and Wait - guarantee that when a process requests a

resource, it does not hold other resources.No Preemption

• If a process that is holding some resources requests another resource that cannot be immediately allocated to it, the process releases the resources currently being held.

Circular Wait• Impose a total ordering of all resource types.